1. Field of the Invention
The present invention relates to a projection system, and more particularly to a projection system in which three projecting lenses can be driven integrally so as to match three beam images on a screen and to focus the images on the screen simultaneously, thereby avoiding a keystoning phenomenon.
2. Description of the Prior Art
Generally, in a three beam splitting-type projection system, three projectors corresponding to red, green and blue colors are placed at predetermined distances away from a screen. A projecting object is projected on a screen through each projector and the images obtained therefrom are overlapped to provide a viewer with a full color image.
In the three beam splitting-type projection system, when the distances between the screen and the projectors and between the projectors themselves are not exactly adjusted, the images projected through the projectors could not be matched.
FIG. 1 is a schematic view of the constitution of a conventional projection system. The system has first, second and third projecting units 1a, 1b and 1c for projecting red, green and blue color beams.
The first, second and third projecting units include first, second and third light sources 4a, 4b and 4c such as xenon lamps, first, second and third spherical reflecting mirrors 3a, 3b and 3c for reflecting the light emitted backwards to the forward direction, first, second and third collimating lenses 5a, 5b and 5c for collimating the forwardly emitted and reflected light and for convening the collimated light into parallel light, first, second and third liquid crystal display (LCD) panels (or slide films) 6a, 6b and 6c, first, second and third projecting lenses 7a, 7b and 7c, etc.
Conventionally, first, second and third projecting units 1a, 1b and 1c are driven independently to adjust the reproduced image on a screen S1. That is, a first image is obtained on screen S1 by moving the focus of the image of one projecting unit among the three projecting unit, vertically and laterally. Then, a complete image is obtained by moving the remaining projecting units vertically and laterally.
In the three beam splitting-type projection system, in case the images projected on screen S1 through each projecting unit do not coincide or in case the focus of the image is not accurate due to inappropriate distances between the screen and each projecting unit, the projection direction and the projection angle should be adjusted by moving each projecting unit with respect to both the screen and the remaining projecting units to compensate for the deviation.
Accordingly, the operation is very complicated and inconvenient. In addition, since the adjusting operation is implemented manually while staring at the image reproduced on screen S1, an accurate adjustment is very difficult if the operator is not an expert.
Moreover, in the above-mentioned three beam projection system, a second light axis Lb of second projecting unit 1b coincides with a central axis L1 of screen S1, while first and second light axes La and Lc of first and third projecting units 1a and 1c are slanted at a predetermined angle to central axis L1 of screen S1. This generates differences in distances when the lights arrive at the left and the right of screen S1 through projecting lenses 7a and 7c. Accordingly, the ratios of image magnification at the left and the right of the screen become different. Consequently, the projected images obtained through first and third projecting lenses 7a and 7b shows vertical trapezoidal distortion to give three disagreeing images, as shown in FIG. 9A. The appearance of the distortion in the image is called as the keystoning phenomenon.
When employing such a conventional projector in the three beam projection system, a disadvantage of increasing the fatigue of a viewer's eyes in viewing the images due to the generation of the keystoning phenomenon occurs.
U.S. Pat. No. 5,125,733 (issued to Lee) discloses a stereoscopic projector for driving a pair of projecting lens holders integrally as a method for overcoming the above-mentioned problems. In this patent, the light axes of the projecting objects and the light axes of the projecting lenses are parallel to the central axis of the screen to coincide two projected images on the screen, thereby preventing the keystoning phenomenon.
Although a good image without the trapezoidal distortion could be obtained by using the above-mentioned apparatus, the apparatus is complicated and the manufacture thereof is not so advantageous.